To me the real problem is that we are not yet considered a breed, which means that we do not now nor have we ever had a type standard. Yes, we have a standard of perfection, but I feel that's more about balance and good basic conformation than a specific type. That said, it's hard to determine then what "Classic" would be. After all, many horses that were NOT stockier did exist in the earlier years too (like Buckeroo, Rowdy, etc).
I would like to disagree with you a bit Leanna. I don't feel that having different types within the breed is moving backward, especially since variety is one of the main selling points of minis. Quarter Horse breeders put as much time and effort into their breeding programs as Arabian breeders, and I think they would be offended to hear someone saying that because their breed standard is less refined they are not improving their horses. Your statement assumes that the current type is the future of minis as a breed, but they've already shuffled through several other types, and I am sure the trends will change again down the road. Until we do become a breed and set an official type standard (and if we did I would gladly adjust my program to meet those standards), I don't see anything wrong with breeding for what you like (chances are if you like it there are others that do too) as long as it's a good and true representation of its type.
To tie this back to the original question, I think it would be very difficult to differentiate between the "Classic" and "Modern" type in minis when there is no historical basis for either. I can understand how people may feel that their heavier horses are not getting their fair due because of preference for a more refined type, but unfortunately that's what showing is--the judges' preference, which in turn is usually determined by what they intuit to be the breed's preference at the time.